Because of the cold weather, we had some troubles with drying out of printed solder paste on the pcb's. The relative humidity in the air declined below of 20%. We have a process window of store the printed board for maximum 24 hours, but with such low moisture levels we have problems with tackiness of the solder paste. Now we use an air humidifier to increase the moisture level to 30% rel. But which is the maximum level to use in a production to assemble msl-3 components?

3 Printer Area Conditions * As stated earlier, heat and humidity are damaging to solder paste. Ideally, the printing area should be maintained at 40% - 50% relative humidity and 72°- 80°F (22-26°C). In addition, no air (cool or warm) should blow directly on the top or bottom of the stencil, as this can cause paste dry-out. It should be noted that some screen printers that run external environmental controllers might circulate air across the stencil surface. Through simple modification this air movement across the stencil can be dramatically reduced or eliminated, which can have a significant impact on the stencil life of the solder paste in use. [AIM Tech-Sheet, Solder Paste Handling Guidelines] * Humidity can greatly affect the stencil life of solder paste. At low humidity [less than 25% relative humidity (RH)], solder paste dries out quickly, increasing viscosity over time. Increasing viscosity will reduce printability and increase the presence of insufficient solder joints. At high humidity (greater than 75% RH), pastes can become hygroscopic, accepting water. Water may produce unwanted reactions within solder paste fluxes, often leading to detects such as poor wetting and solder balls. [Brian Smith, Kester Northrop Grumman, bsmith@kester.com] * The viscosity of solder paste is highly dependent on its temperature. Paste viscosity will decrease as temperature increases. At temperatures above 25*C, the paste may become runny and produce bridging or other defects. At temperatures below 21*C, paste viscosity will increase, resulting in sluggish paste flow and insufficient solder joints. For most solder pastes, the viscosity increases by 4 to 6% for each degree of temperature decrease, so maintaining good control of the environment is important. [Tony Longo, Kester Northrop Grumman, tlongo@kester.com] * The recommended ambient conditions for application of the product by printing or dispensing are 22-28°C and 30-70% relative humidity. There are many types of printing equipment, squeegees and combinations of stencil design and process conditions. Therefore the following recommendations should be considered as guidelines for the initial setting of the process. If the solder cream has been prepared properly, it will not be necessary to shear thin the cream prior to printing the first board. [User's Guidelines Cobar No-Clean Lead-Free, Tin-Based Solder Paste X- Series] * Working Environment: Solder Paste performs best when used in a controlled environment. Maintaining an ambient temperature of between 20 and 25°C at a relative humidity of less than 55% will ensure consistent performance and maximum life of the paste. [Multicore Solder Paste Handling Guidelines, September 2004] * As stated earlier, heat and humidity are damaging to solder paste. Ideally, the printing area should be maintained at 40% - 50% relative humidity and 72°- 80°F (22-26°C). In addition, no air (cool or warm) should blow directly on the top or bottom of the stencil, as this can cause paste dry-out. It should be noted that some screen printers that run external environmental controllers might circulate air across the stencil surface. Through simple modification this air movement across the stencil can be dramatically reduced or eliminated, which can have a significant impact on the stencil life of the solder paste in use. [AIM Tech-Sheet] *

MSL 3 means you can have the devices open for a maximum of 168 hours in an atmosphere of less then or equal to 30 C and 60% RH. If the humidity is less then 60% then it means the devices will absorb less moisture in a given time, but even at 40% RH you have to make sure the devices do not exceed their floor life too much. For moisture sensitive devices, DRYER IS BETTER...but this can cause paste issues as you have seen and ESD issues. Moisture Sensitive devices should be stored and handled as per IPC JEDEC 33B01.

With lower Humidity levels, we expose to greater risk towards ESD related failures seen on components. Pl. ensure to have your entire work area fully guarded against ESD and maintain proper material handling instructions across the entire flowline until final packaging before warehousing / despatch.